DESCRIPTION: (from the applicant's abstract) This is an application for funds to perform experiments directed toward understanding the mechanism by which "viral protein U (Vpu)" mediates the exit of HIV-1 particles from cells. The proposed experiments are centered around the role of a novel cellular protein that interacts directly with Vpu in vitro and in human cells. This novel cellular protein "U-binding protein (Ubp) is a member of a protein superfamily that includes the immunophilins and a class of serine/threonine phosphatases containing tetratricopeptide repeats (TPRs). The applicants have several complementary goals. In the first specific aim experiments are proposed to examine the interaction between Vpu and Ubp in more detail. In particular, the applicant will identify regions of the two proteins that are required for protein:protein interaction, isolate mutants that exhibit altered binding stability, and test the hypothesis that Vpu action is mediated via Ubp. The applicants' preliminary data also indicate that Ubp interacts directly with HIV-1 Gag protein in vitro, and with the capsid domain of HIV-1 Gag in human cells. Thus, experiments proposed in specific aim 2 will identify the sites on Ubp and Gag that are required for this interaction. In the experiments proposed in specific aim 3, the applicants will examine the effect of expression of wild-type and mutant forms of Ubp on particle release. Similarly, the applicants may determine whether ablation of endogenous Ubp expression affects the efficiency of particle release. Ubp has not been previously characterized. In experiments proposed in the fourth specific aim the applicants will examine the intrinsic properties of Ubp and try to gain insight into the normal role of Ubp in the cell. The overall goal of the proposed study which is to develop a comprehensive picture that accounts for the molecular mechanism by which Vpu mediates virus particle release.